A typical mining shovel includes a turntable mounted on a crawler truck, and supporting an A-frame and a cab. A boom extending from the turntable has an upper end supported by the A-frame. The boom pivotally supports a dipper handle which pivots in a vertical plane. A dipper fixed to a distal end of the dipper handle is raised and lowered by a hoist cable which extends over a sheave at the top of the boom and down to a padlock on the dipper. The hoist cable provides for the vertical, raising and lowering, movement of the dipper. A crowd mechanism extends and retracts the dipper handle to provide the horizontal component, or crowd, of the dipper's movement.
To provide the crowd mechanism for extending and retracting the dipper handle, many different types of crowd mechanisms have been developed. Prior art systems include rack and pinion crowd mechanisms and rope crowd mechanisms. Rack and pinion systems include a rack fixed to the dipper handle that engages a rotatably driven pinion, or gear, mounted in the boom. Rope crowd mechanisms include metal ropes that are wound and unwound from a crowd drum to extend and retract the dipper handle. While these types of crowd control mechanisms are advantageous in certain respects, they also suffer from certain disadvantages. For example, the rack teeth and ropes are prone to break when excessive force is applied Furthermore, the handles in rack and pinion mechanism which must remain rotatably fixed, and therefore cannot be rotated.
Hydraulic crowd mechanisms, such as the mechanism disclosed in U.S. Pat. No. 3,425,574, are also known. These hydraulic systems typically utilize a large double-acting hydraulic actuator for extending and retracting the dipper handle. Hydraulic crowd mechanisms provide certain advantages over other types of systems because the hydraulic systems can use round tubular handles that are free to rotate. Further, they do not include rack teeth or ropes, which are prone to break when excessive force is applied While hydraulic mechanisms therefore provide certain advantages over other types of crowd controls, particularly with respect to maintenance, the volume of hydraulic fluid necessary to control the crowd of a mining shovel dipper handle requires complex large hoses, designed to carry a large volume of fluid, and at high pressures. Because of the high pressure application, these hoses are subject to significant wear, and requiring a significant degree of maintenance. Accordingly, a need exists for a hydraulic crowd mechanism which responds quickly to operator inputs, and which operates without a significant degree of maintenance.